Light Emitting Diode

Light emitting diodes (LEDs) are one of the most efficient lighting technology of today, and nearly used in all electronic lighting applications. A light-emitting diode (LED) is a semiconductor based lighting system, which emits light when electrical energy is applied to it. These tiny bulbs are illuminated solely by the movement of electrons in a semiconductor material. The lifespan of an LED is much greater than that of an incandescent bulb.

Working

Early in 1960s, early LEDs emitted low-intensity red light, and today we have LEDs available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. When a light-emitting diode is forward-biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electron-luminescence and the color of the light is determined by the energy gap of the semiconductor. This is how light is emitted from an LED.

How is light displayed?

The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. In typical diodes, current flows easily from the p-side(anode), to the n-side(cathode), but not in the reverse direction. Charge-carriers — electrons and holes — flow into the junction due to difference in voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The wavelength of the light emitted, and thus its color depends on the band gap energy of the materials forming the p-n junction.

LED Colors

Wavelength
(nm)

Color Name

Fwd Voltage
(Vf @ 20ma)

Intensity
5mm LEDs

Viewing
Angle

LED Dye Material

940

Infrared

1.5

16mW
@50mA

15°

GaAIAs/GaAs --
Gallium Aluminum Arsenide/Gallium Arsenide

880

Infrared

1.7

18mW
@50mA

15°

GaAIAs/GaAs --
Gallium Aluminum Arsenide/Gallium Arsenide

850

Infrared

1.7

26mW
@50mA

15°

GaAIAs/GaAs --
Gallium Aluminum Arsenide/Gallium Aluminum Arsenide

660

Ultra
Red

1.8

2000mcd
@50mA

15°

GaAIAs/GaAs --
Gallium Aluminum Arsenide/Gallium Aluminum Arsenide

635

High
Eff. Red

2.0

200mcd
@20mA

15°

GaAsP/GaP - Gallium
Arsenic Phosphide / Gallium Phosphide

633

Super
Red

2.2

3500mcd
@20mA

15°

InGaAIP - Indium
Gallium Aluminum Phosphide

620

Super
Orange

2.2

4500mcd
@20mA

15°

InGaAIP - Indium
Gallium Aluminum Phosphide

612

Super
Orange

2.2

6500mcd
@20mA

15°

InGaAIP - Indium
Gallium Aluminum Phosphide

605

Orange

2.1

160mcd
@20mA

15°

GaAsP/GaP - Gallium
Arsenic Phosphide / Gallium Phosphide

595

Super
Yellow

2.2

5500mcd
@20mA

15°

InGaAIP - Indium
Gallium Aluminum Phosphide

592

Super
Pure
Yellow

2.1

7000mcd
@20mA

15°

InGaAIP - Indium
Gallium Aluminum Phosphide

585

Yellow

2.1

100mcd
@20mA

15°

GaAsP/GaP - Gallium
Arsenic Phosphide / Gallium Phosphide

4500K

"Incan-
descent"
White

3.6

2000mcd
@20mA

20°

SiC/GaN -- Silicon
Carbide/Gallium Nitride

6500K

Pale
White

3.6

4000mcd
@20mA

20°

SiC/GaN -- Silicon
Carbide/Gallium Nitride

8000K

Cool
White

3.6

6000mcd
@20mA

20°

SiC/GaN - Silicon
Carbide / Gallium Nitride

574

Super
Lime Yellow

2.4

1000mcd
@20mA

15°

InGaAIP - Indium
Gallium Aluminum Phosphide

570

Super
Lime Green

2.0

1000mcd
@20mA

15°

InGaAIP - Indium
Gallium Aluminum Phosphide

565

High
Efficiency
Green

2.1

200mcd
@20mA

15°

GaP/GaP - Gallium
Phosphide/Gallium Phosphide

560

Super
Pure Green

2.1

350mcd
@20mA

15°

InGaAIP - Indium
Gallium Aluminum Phosphide

555

Pure
Green

2.1

80mcd
@20mA

15°

GaP/GaP - Gallium
Phosphide/ Gallium Phosphide

525

Aqua
Green

3.5

10,000mcd
@20mA

15°

SiC/GaN - Silicon
Carbide / Gallium Nitride

505

Blue
Green

3.5

2000mcd
@20mA

45°

SiC/GaN - Silicon
Carbide / Gallium Nitride

470

Super
Blue

3.6

3000mcd
@20mA

15°

SiC/GaN - Silicon
Carbide / Gallium Nitride

430

Ultra
Blue

3.8

100mcd
@20mA

15°

SiC/GaN - Silicon
Carbide / Gallium Nitride

Power supply for LEDs

The signal, in its natural form is always oscillating. If this signal is fed to LED, it fluctuates and eventually burn out. It needs a regulated (constant) voltage. If it has to be connected to 230/420V AC supply, it has to be regulated using voltage regulators. For power LEDs which require high current, LED drivers are used for current boosting.

Advantages:

Size: Size of LED is usually very small (smaller than 2 mm sq.) and hence they are easy to mount on PCBs.

Switching: Switching refers to the ON/OFF cycles. LEDs have very large switching cycles-it can be turned on and off millions of times before burning.

Efficiency: LEDs produce more light per watt than incandescent bulbs.

Focus: The LED can focus its light in a particular direction.

Shock resistance: LEDs, being solid state components, are difficult to damage with external shock.

Toxicity: LEDs do not contain mercury, lead or any other toxic elements.

Color: LEDs can emit light of any required color with the change of the semiconductor material.

Brightness: Brightness of an LED can be easily controlled by varying the duty cycle of the pulse by the method called Pulse Width Modulation (PWM).

Switching time: Switching time of an LED is extremely low. RF LEDs have switching time in pico seconds.

Disadvantages:

Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment.

Safety: It is now evident that some colors of LEDs emit light which exceeds the Eye standards and hence it may pose a threat to human eye in long term.

Voltage sensitivity: LEDs must not be supplied voltage abouve its upper threshold limits. On doing so, it immediately burns out.

Initial cost: If cost efficiency is considered, price per lumen, of an LED is high.

Applications for LEDs

Display lighting in banners and street advertisments

Back lighting in televisions and mobile phones

Sign lighting used as street indicators

Traffic lights and signals

Light source for machine vision systems

Automobile lights used in cars and trucks

Toys and and entertainment systems

Flashlights, glow-lights, and lasers.

Elevator Push Button Lighting

Red or yellow LEDs are used in indicator and alphanumeric displays in environments

Red, yellow, green, and blue LEDs can be used for model railroading applications

In optical fiber and Free Space Optics communications.

In dot matrix arrangements for displaying messages.

Interfacing LED with Arduino Microcontroller

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